The technology disclosed here relates to a fuel cell system having a humidified oxidant flow.
The performance and efficiency of a fuel cell can be increased by humidifying the cathode air. This is particularly the case at increased operating temperatures of the fuel cell, e.g. at a coolant output temperature from the fuel cell of greater than 75° C. Additional humidification of the oxidant flow is particularly necessary at increased operating temperatures and/or with high fuel cell capacities.
DE 102008053151 A1 discloses a device in which a liquid medium may be introduced upstream of a charge air cooler and downstream of a compressor by means of a metering device. The humidification level here is set by a charge air cooler acting as a contact humidifier and by an additional membrane humidifier. It has been established that failure of the compressor occurs more frequently in fuel cell systems having humidifying devices. It is furthermore found in the previously known systems that, when the water is introduced into the supply line, the water does not completely evaporate. Furthermore, liquid water can form again from evaporated water on the walls of the supply line. In winter, this liquid water can freeze in the supply line. Water defrosting in the supply line of the fuel cell during a cold start can markedly impair the cold start. The frozen liquid water can furthermore damage the supply line.
It is the object of the present invention to reduce or eliminate the disadvantages of the previously known solutions. Further objects emerge from the advantageous effects of the technology disclosed here. The object(s) is/are achieved by the fuel cell system in accordance with embodiments of the invention.
The technology disclosed here relates to a fuel cell system having at least one fuel cell. The fuel cell system is intended, for example, for mobile applications such as motor vehicles. In its simplest form, a fuel cell is an electrochemical energy converter, which converts fuel and oxidant into reaction products and thereby produces electricity and heat. The fuel cell comprises an anode and a cathode which are separated by an ion-selective separator. The anode has a supply for supplying fuel to the anode. Preferred fuels are: hydrogen, low-molecular alcohol, biofuels or liquefied natural gas. The cathode has, for example, a supply for oxidant. Preferred oxidants are, for example, air, oxygen and peroxide. The ion-selective separator can be formed, for example, as a proton exchange membrane (PEM). A cation-selective polymer electrolyte membrane is preferably used. Materials for such a membrane are, for example: Nafion®, Flemion® and Aciplex®. A fuel cell system comprises at least one fuel cell and peripheral system components (BOP components) which can come into use during operation of the at least one fuel cell. A plurality of fuel cells are generally combined to form a fuel cell stack.
The fuel cell system comprises an oxidant conveyor which conveys an oxidant to the cathode of the fuel cell. Such an oxidant conveyor can be formed, for example, as a supercharger or turbo compressor or as a compressor. The oxidant conveyor can convey the oxidant, preferably oxygen or air, through a cathode supply line, a supply line or an intake tract (the term “supply line” will be used below for simplicity) into the cathode. The oxidant conveyor is, for example, capable of compressing the oxidant to a pressure of greater than 1 bar. With this, the oxidant can be heated, for example, up to 160° C. and above. The oxidant conveyor is arranged upstream of the cathode in the supply line.
The fuel cell system furthermore comprises at least one humidifier, which introduces, preferably injects or sprays, a liquid, in this case water, into the oxidant flow. The terms spraying or atomizing of the liquid refer here to the dispersal of the liquid into the finest droplets (aerosol or mist) in the oxidant. By atomizing the water, a good mixing with the air flow can be ensured and the injected water can evaporate or vaporize more quickly owing to the large reactive liquid surface. To this end, the fuel cell system can be equipped with pressure nozzles which inject the water into the oxidant flow. For the finest droplets, for example, an injector or nozzle is provided with a water pressure of up to 20 bar. The nozzles can also be designed in such a way that the oxidant flow takes in the water itself in accordance with the Venturi effect or according to the principle of the ejector pump. The humidifier is arranged in and/or downstream of the oxidant conveyor and upstream of the fuel cell, preferably upstream of a heat exchanger or heat interchanger (hereinafter: heat exchanger) in the supply line. The water which is injected constitutes a store for a cooling capacity. To this end, a water reservoir can store the accumulating product water, for example, which is separated, for example, from the exhaust gas of the fuel cell.
The humidifier is preferably arranged at a spacing from the heat exchanger. The humidifier is preferably arranged in (e.g. in the volute) and/or adjacent to the oxidant conveyor. The humidifier is particularly preferably arranged at a spacing of at least about 0.3L, furthermore preferably at least about 0.5L and particularly preferably at least about 0.75L from the heat exchanger or from the fuel cell, wherein L is the distance of the oxidant flow between the oxidant conveyor and the heat exchanger. The humidifier is preferably formed integrally with the oxidant conveyor. It is thereby advantageously achieved that at least a high proportion of the water sprayed in is evaporated before it enters the heat exchanger.
The fuel cell system disclosed here comprises at least one water sink. The water sink is arranged in particular between the at least one fuel cell and the oxidant conveyor.
It has been established that unfavorable geometric integration, dynamic operation (constant transverse/longitudinal acceleration) or tilting can cause the liquid water to run back into the compressor and damage it. The water sink can be formed and arranged in the supply line in such a way that it prevents liquid water located in the supply line from flowing back or to or into the oxidant conveyor.
Such a water sink can also be referred to as a return stop. In particular, the water sink can be a depression or protrusion in relation to other regions of the supply line. The depression or protrusion can be arranged lower or deeper than the surrounding regions of the supply line. Any liquid water which is located in the supply line (e.g. on the walls of the supply line) collects in the water sink. The water sink is therefore formed and arranged in such a way that the water sink can collect or store liquid water located in the supply line. The water sink is preferably arranged at the deepest point of the supply line section for which the water sink is intended to collect liquid water. The water can thus be prevented from returning to the oxidant conveyor. It can thus be ensured that the liquid water is unable to cause mechanical and/or electrical damage in the oxidant conveyor. The water sink is furthermore preferably formed in the supply line in such a way that the flow cross-section for the oxidant flow O is not altered by the water sink. In other words, the water sink is formed by a depression or protrusion in the supply line wall.
The term “adjacent to the oxidant conveyor” comprises embodiments in which the humidifier or the water sink is arranged immediately adjacent to the oxidant conveyor or at a slight spacing from the oxidant conveyor. The at least one water sink is preferably formed adjacent to the oxidant conveyor. The humidifier and/or the water sink is preferably arranged to be a maximum of about 0.2L, furthermore preferably a maximum of about 0.1L and particularly preferably a maximum of about 0.05L away from the oxidant conveyor.
The at least one water sink is preferably fluid-connected to the humidifier and/or to at least one further water injection device. It is thus ensured that the collected liquid water can be discharged again by simple means. The liquid water is thus efficiently used again for humidifying the fuel cell. For example, the water sink can be connected to the at least one humidifier via corresponding lines. However, a design in which the water sink is fluid-connected only to a water injection device arranged immediately adjacent to the water sink in the supply line is preferred. The spacing between the water sink and the water injection device is preferably a maximum of about 0.2L, furthermore preferably a maximum of about 0.1L and particularly preferably a maximum of about 0.05L. It is thus advantageously possible to dispense with any space-consuming and costly lines.
The at least one water injection device is preferably formed as an ejector pump. Ejector pumps as such are known. They substantially constitute a rotated pitot tube which can itself take in water in accordance with the Venturi effect, for example. Such ejector pumps are also referred to as ejectors or as jet pumps. The oxidant flowing past the ejector pump can serve as a propellant here and has the expedient effect that the ejector pump takes in liquid water. Such an embodiment is particularly advantageous since the ejector pump can be designed such that it does not require external (e.g. electrical or pneumatic) energy. It is particularly economical and robust and moreover needs very little installation space.
The water injection device or ejector pump is preferably formed in such a way that it can introduce the liquid water collected in the water sink into the oxidant flow. The liquid water is thereby introduced directly into the turbulent part of the oxidant flow from the collecting reservoir as a result of the low static pressure, whereby increased atomization is effected. A film is prevented from forming on the wall and complete emptying of the water sink can also be ensured with lower flow rates.
The fuel cell system preferably has at least one adjusting mechanism. The adjusting mechanism can be formed to alter the position of the water injection device in the supply line. In particular, the adjusting mechanism can be suitable for conveying the water injection device into a first or lowered position, in which the water injection device impedes the oxidant flow to a lesser extent than in a second or raised position assumed by the water injection device in the event that at least some water has collected in the water sink. In the second position, the water injection device is capable of injecting liquid water into the oxidant flow.
The adjusting mechanism preferably includes a float which is fastened to the water injection device. Furthermore, the adjusting mechanism preferably includes a joint which connects the water injection device rotatably to the supply line.
The fuel cell system can have a plurality of water sinks. The at least one water injection device can be fluid-connected to the plurality of water sinks, for example via corresponding water lines. At least one, preferably all, of the plurality of water sinks can each include a closing device. The closing device can be formed to interrupt the fluid connection between the water sink and the at least one water injection device. In the event of a non-local reintroduction of the liquid water of one or more sinks, a closing device, for example a structure actuated by a float, is advantageous for closing the water lines or intake lines which are not required in each case. The intake capacity of the water injection device can thus be focused on the water sinks which are actually storing liquid water. Parasitic air flows without a liquid water fraction are prevented or reduced. A common water injection device for a plurality of water sinks, even across various functions (return prevention/stopping action in the fluid direction), can be realized by passive water or intake lines to the respective water sinks.
With the technology disclosed here, it is possible to prevent damage to components and at the same time to achieve improved atomization and better efficacy of the original water injection through the efficient reintroduction of returning waste water. The system operates predominantly with passively actuated components and is therefore very reliable.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.